This invention relates to the hydrometallurgical oxidation of sulfidic minerals, more particularly copper minerals, utilizing an aqueous leaching slurry and nitrogen dioxide gas as an oxidant. More specifically, it relates to an energy efficient sulfidic mineral leaching procedure employing a high slurry density leach liquor.
The conventional high temperature oxidation of sulfides has the advantages of being relatively non-selective in terms of mineral reactivities and of consuming much less energy during the oxidation process as compared with most hydrometallurgical oxidation processes. Unfortunately, roasts of this type produce large volumes of relatively dilute sulfur dioxide off-gas which, in order to comply with environmental protection standards, must be collected, scrubbed, etc. by relatively expensive ancillary equipment. However, despite this disadvantage, high temperature oxidation processing of sulfidic minerals has been preferred in the art because the hydrometallurgical processes are even more expensive.
The relatively high cost of nitric acid containing hydrometallurgical processes is due to several factors. First, if oxidant regeneration techniques are not employed, large volumes of liquid are required in order to supply sufficient nitric acid in a sulfuric acid-nitric acid leach liquor to generate an acceptably high concentration of soluble metal values. This also results in a considerable amount of wasted nitrate. Furthermore, the kinetics of the conventional nitric-acid leach are such that large amounts of power are consumed to maintain the slurry suspension, to preheat and cool the slurry, and to move about the large volumes of leach liquor used. While the use of a nitric-acid plant to regenerate the oxidant minimizes loss of nitrate in such processes, it also increases the capital cost of the system. Another expense associated with conventional nitric-acid hydrometallurgical leaching procedures is the cost of collecting and regenerating relatively large volumes of off-gas which are produced as a reaction by-product. The capital expense involved with capturing, scrubbing, recycling, etc. the off-gas significantly contributes to the capital cost of the process.
Copending application Ser. No. 855,983 to T. C. Frankiewicz et al., filed on even date herewith, entitled "Copper Leaching Process Employing Nitrogen Dioxide" discloses an improved hydrometallurgical oxidation procedure wherein nitrogen dioxide is used as an oxidizing agent in an aqueous acidic leach liquor. The teachings of the foregoing application are incorporated herein by reference. While the procedure disclosed in that application is characterized by many advantages, it nevertheless suffers from the high-energy cost associated with mixing and moving about relatively large volumes of low slurry density leach liquors. Furthermore, as is true with most such leaching procedures, all the metal values which react become solubilized in a single aqueous phase, requiring a down-stream separation technique for, e.g., separating iron, nickel, copper, etc. values from each other.
Many of the disadvantages of aqueous sulfide oxidations would be eliminated if the oxidation reaction rates could be improved and the normally required use of large liquid dilution volumes were circumvented. Such a process would be ideal and could effectively compete with high temperature oxidation if, in addition to the foregoing advantages, relatively little off-gas was produced which had to be captured and treated for recycle.